It commonly is thought that replacing incandescent lamps with compact fluorescent lamps (CFLs) always results in substantially lower electricity consumption and utility costs. CFLs are advocated widely for commercial use with the understanding that there will be large energy and energy-cost savings. However, this usually is not the case.
Let us assume that CFLs use one-fourth of the electric energy that incandescent lamps use for the same amount of light output. Also, let us assume that electricity costs 10 cents per kilowatt-hour and natural gas costs $2 per therm, which are common prices in many parts of the United States. Further, let us assume that electric resistance heating is 100-percent efficient and that gas heating in most existing buildings is 68-percent efficient, not including any additional electricity that might be used for fans or pumps to deliver heat to the building. Small changes in these assumptions will not affect the conclusions in this column materially. The concepts in this column apply to all lighting retrofits that reduce lighting wattage.
In residential buildings that are fully air-conditioned around the clock during summer, CFLs produce less heat and require less cooling energy and, therefore, less electricity than incandescent lamps. In residential buildings that are not cooled during summer, CFLs produce less heat and use less electricity than incandescent lamps, but do not save cooling energy because none is used. Also, keep in mind that lighting is used less in summer than in winter because the daylight hours are longer.
Surveys about residential cooling show some interesting statistics. Many houses and apartment buildings have window or wall air conditioners, but only some use them all summer. Most of those with window or wall units use them only a few times. While the majority of residences have central air-conditioning systems, less than half use them all summer, and many use them only a few times. Only when CFLs are operated at the same time as air conditioners will they use less cooling energy than incandescent lamps.
Data show that 93 percent of commercial floor space is heated. When generating the same amount of light, incandescent lamps produce four times more heat than CFLs. Therefore, incandescent lamps require four times less heating energy than CFLs. Actually, 100 percent of the electricity used in any lamp (and ballast) — including that used for light — ends up as heat. Electric resistance heating produces these same results.
At 10 cents per kilowatt-hour and 100-percent efficiency, electricity costs $29.30 per million British thermal units delivered to a space. At $2 per therm and 68-percent efficiency, the delivered cost of natural-gas-generated heat to a space is $29.40 per million British thermal units. Thus, with energy costs of 10 cents per kilowatt-hour and $2 per therm, replacing incandescent lamps with CFLs saves virtually no money during winter. Utilizing CFLs with electric resistance heating will not produce energy-cost savings during winter either because less lighting-generated heat will be replaced by the identical amount of resistance heat.
The only energy-cost savings associated with the use of CFLs would be during summer, if the delivered costs of heating energy are close to the costs of electric energy and the space being served is not cooled or minimally cooled. In the unusual situation in which electricity cost 20 cents per kilowatt-hour and natural gas cost $1 per therm, using CFLs during winter would provide some energy-cost savings. If the delivered cost of natural gas per British thermal unit exceeded the cost of electricity per British thermal unit, using CFLs during winter would cost more than using incandescent lamps. For example, with gas prices at $2 per therm and electricity below 10 cents per kilowatt-hour, using CFLs during winter would result in higher energy costs.
In most cases, except with extensive cooling use in warm climates, replacing incandescent lamps with CFLs can result in a modest energy-cost savings on an annual basis. When utilizing CFLs, most or all of the energy-cost savings occur during summer. When utilizing CFLs during winter, some or all energy-cost savings are replaced with additional heating fuel costs.
CFLs and other more-efficient lamps save energy and energy costs during summer and even more when cooling is used simultaneously with lighting. CFLs and other more-efficient lamps may or may not save energy and energy-costs during winter, depending on the relative costs of electricity and heating energy. In almost all cases, CFLs and other more-efficient lamps will not save nearly as much energy and energy costs as claimed.
A consulting engineer focusing on energy management, procurement, and problem solving, Larry Spielvogel, PE, is an ASHRAE Fellow, an ASHRAE Distinguished Lecturer, a recipient of an ASHRAE Distinguished Service Award, and a past ASHRAE director at large. He chaired the panel for the original Standard 90.1 on HVAC systems and, from 1999 to 2002, chaired the Standard 90.1 project committee. He is a registered professional engineer in 49 states, as well as a chartered engineer in England and a European engineer in all common-market countries.
Most commercial electricity rates impose premium prices for low power factors because they increase power-transmission losses and necessitate greater electrical infrastructure. Most CFLs have a very low power factor. Energy Star-qualified CFLs are required to have a minimum power factor of 0.5, which is sufficient to impact most commercial and industrial electricity costs. Incandescent lamps have a unity power factor and, thus, do not affect the power-factor provisions of commercial and industrial electricity rates.